update types/aws-lambda.d.ts from DefinitelyTyped and ease that maintenance

We inline TypeScript types for an AWS Lambda handler for our `apm.lambda()` wrapper API method, in https://github.com/elastic/apm-agent-nodejs/blob/master/types/aws-lambda.d.ts. That is pulled from https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/aws-lambda/handler.d.ts#L99 -- or at least that is where it lives now. I'd have to dig back to see where it originally came from.

The layout of our aws-lambda.d.ts and the community-maintained handler.d.ts are a little different. It would be nice to:

1. Update to the latest from DefinitelyTyped. The recent https://github.com/elastic/apm-agent-nodejs/pull/2439 shows that there are some differences. There are some other differences as well.
2. If possible, have our aws-lambda.d.ts structure/layout match the file from DefinitelyTyped to make maint/updating easier.
3. Is there some mechanism we can have that will automatically check for diffs, or watch for upstream changes?